1. Field of the Invention
The present invention relates to optical interferometers. Interferometers are used in many fields but the present invention is particularly, though not exclusively, concerned with interferometers whose applications include the detection of optical phase changes due to surface binding under surface plasmon resonance (SPR) conditions and to the detection of binding at multiple, discrete sites in the surface using the phase images thereby obtained.
2. Description of the Related Art
Surface Plasmons (SP) are collective oscillations of free electrons constrained to move in a thin film of a conductor considered as a two-dimensional environment. These oscillations are excited by external electromagnetic radiation coming from a high refractive index medium. For a given configuration there exist a relatively small range of film thickness for which resonance is supported. Film thickness outside of this range are non-resonant. Surface plasmons also have a characteristic propagation length that limits fundamentally the spatial resolution achievable with this technique.
SPR is used in molecular binding detection analysis where typically molecules to be tested are deposited on the thin film and potential binding agents are passed in a solution or gas over the rear face of the thin film. If binding occurs between a molecule and a binding agent under SPR conditions the refractive index at that point will change and can be detected.
Uses of molecular binding detection analysis include, but are not limited to, measuring the expression levels of genes and proteins in biological samples, determining the functions of genes and proteins, identifying actual or potential therapeutic drugs and other molecules, determining the biological effects of actual or potential therapeutic drugs and other molecules. These applications can be used in biological research activities, in discovery and development of drugs, and as diagnostic tests.
In prior art a Mach Zehnder device has been configured to measure SPR phase changes produced by variations in the refractive index of a gas local to the measurement surface. A major disadvantage of this type of device, particularly with respect to routine laboratory use, is that it requires four independent components: two beam splitters and two mirrors, one of which is the resonant surface in the case of the SPR configuration. As a result, the arrangement is relatively bulky and, more critically, the output is sensitive to sub-wavelength relative displacements of these components and hence very small mechanical and environmental perturbations.